<!DOCTYPE html>
<html lang="zh_cn">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        h1 {
            color: red;
        }

        h1 {
            color: blue;
        }

        p {
            color: blue;
        }

        .box {
            width: 350;
            height: 150;
            border: 5;
        }

        html {
            box-sizing: border-box;
        }

        *,
        *::before,
        *::after {
            box-sizing: inherit;
        }
    </style>
</head>

<body>
    <h1>This is my heading.</h1>
    <p>text in p <span>text in span</span></p>
    <div class="box"></div>
</body>

</html>